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Field Service & Trade Estimators · HVAC Engineering

Free HVAC Duct Calculator | Area, Airflow Velocity & Pressure Drop

Calculate HVAC duct cross-sectional area, airflow velocity (FPM), elbow equivalent length, and total static pressure drop (in. w.g.) for round and rectangular ducts.

Duct Parameters

Airflow, Dimensions & Fittings

Results

Duct Sizing & Pressure Drop

Calculated Velocity (FPM) 733.39 FPM Dimensions: 10" Ø Round · Area: 78.54 sq. in.
Area (sq. in.)78.54
Equivalent Length (ft)80.00 ft
Total Pressure Drop (in. w.g.)0.064 in. w.g.
Airflow StatusQuiet (<900 FPM)

Step-by-step guide

How to Use the HVAC Duct Calculator

  1. 1
    Enter required airflow (CFM) and duct shape

    Choose Round (diameter in inches) or Rectangular (width and height in inches).

  2. 2
    Add friction rate and straight duct length

    Enter your design friction rate (in. w.g. per 100 ft, typically 0.08–0.10) and straight run length.

  3. 3
    Include elbows and review velocity & pressure drop

    Select 90° (5 ft), 45° (2.5 ft), or Long-Radius (1.8 ft) elbows to see Equivalent Length and Total Pressure Drop.

Formula & methodology

HVAC Duct Area, Velocity & Pressure Drop Formulas

Round Duct Area (sq. in.) = π × (Diameter ÷ 2)²
Rectangular Duct Area (sq. in.) = Width × Height
Airflow Velocity (FPM) = (Airflow CFM × 144) ÷ Duct Area (sq. in.)
Total Equivalent Length (TEL) = Straight Duct Length + (Number of Elbows × Elbow Factor)
Total Pressure Drop (in. w.g.) = (Friction Rate ÷ 100) × Total Equivalent Length

Common questions

Frequently Asked Questions

What is the recommended airflow velocity (FPM) for HVAC supply ducts?

Residential supply branch ducts typically target 600–900 FPM to keep airflow quiet, while commercial main trunks often run 1,000–1,500 FPM.

Why do elbows add 'equivalent length' to a duct run?

Turning air around a 90° or 45° bend creates turbulence and static pressure loss equivalent to several additional feet of straight sheet-metal duct.

Do you also have a Commercial Kitchen Hood CFM Calculator?

Yes! Check the related tools below for our Commercial Kitchen Hood Exhaust CFM Calculator for NFPA 96 canopy and island hoods.

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